Re: [boost] result_of<F()>::type is void ... why?

From: Daniel Walker (daniel.j.walker_at_[hidden])
Date: 2007-05-21 16:45:42

On 5/21/07, Eric Niebler <eric_at_[hidden]> wrote:
> Daniel Walker wrote:
> > Incidentally, we do know how to test for member templates now. Here's
> > a thread archive discussing my patch to MPL that does this:
> > http://tinyurl.com/2os8fl.
> >
> > So, it would be possible to implement a result_of that deduces the
> > correct return type in corner cases like your example without
> > decltype. But decltype is a much better all around solution.
>
> Can you comment on why some of your tests fail with Comeau? I haven't
> looked into your patch, but if it doesn't work with Comeau, there's a
> good chance it's non-standard, which would be a deal-breaker for
> tr1::result_of.

I've been in touch with the Comeau folks, and they determined that the
Comeau behavior that causes some tests to fail is non-standard. I
believe they're working on fixing it for an upcoming release. So,
Comeau will eventually pass all tests. Actually, this was a couple of
months ago, and they may already have it fixed. I haven't gone back to
check.

But again, rather than changing the heuristic in TR1 to include a
check for a result<> member, I prefer the recent amendment, which
removes section 3.4/3 entirely since implementers can just use
decltype.

http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2194.pdf

Daniel